A ray of light in a first medium of high refractive index directed toward a second medium of a lower index passes into the second medium with refraction, provided the angle of incidence is not too large. As the angle of incidence of the ray increases, a critical angle of incidence is reached at which the ray does not pass into the second medium but is totally reflected at the surface of separation. In general, total reflection occurs at the boundary separating two media having different refractive indices, when any ray in the medium of higher index is directed toward the other medium at an angle of incidence greater than the critical angle.
Seward, U.S. Pat. No. 3,137,794, incorporated herein by reference, provides a directionally sensitive light detector based on this phenomena in which an isosceles prism having isosceles angles custom ground to the critical angle for light of a given wavelength is provided with first and second photocells respectively mounted in a position to intercept light refracted through the side faces of the prism. Light normally incident to the hypotenuse strikes the side face of the prism at the critical angle and is totally internally reflected. Light incident at an angle other than normal to the hypotenuse face strikes one of the side faces at an angle of incidence less than the critical angle whereby the light is refracted through that side face and onto the corresponding photocell. A differential circuit is responsive to the photocell signals and is operative to provide an indication of the direction of the light source off of the optical null of the prism. The presently known directionally sensitive light detectors, however, are comparatively expensive to manufacture, or to procure, since prisms ground and polished at the critical angle for a given wavelength are not commercially produced in quantity. Furthermore, when used as an autocollimator, the known systems produce an undesirable sensitive variation with changes in reflecting surface position, in focus, and with changes in optical and atmospheric parameters.